Williams, Senior Circuit Judge:
Officers of the United States swear to faithfully discharge the duties of the[ir] office. 5 U.S.C. § 3331 ; see also U.S. Const. art. II, § 1, cl. 8 (requiring the President to swear that he will faithfully execute the Office of President). When they fall short, the Mandamus Act offers relief; it vests district courts with jurisdiction over any action in the nature of mandamus to compel an officer or employee of the United States ... to perform a duty owed to the plaintiff. 28 U.S.C. § 1361. The question here is whether the Act extends to duties pertaining not to an officers public office, but to his pre-election (or pre-appointment) acts. We hold that it does not.
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This case concerns a dispute about a then-presidential candidates financial disclosure report. Under the Ethics in Government Act of 1978, candidates for certain offices, including the Presidency, must file financial disclosures with the Federal Election Commission. 5 U.S.C. app. § 103(e). Subject to some limitations, a presidential candidates financial disclosure shall include a full and complete statement with respect to, among other things, the identity and category of the total liabilities owed to any creditor. Id . § 102(a)(4).
Plaintiff Jeffrey Lovitky, appellant here, alleges that, as a presidential candidate, Donald J. Trump filed a financial disclosure report with the FEC on May 16, 2016. Compl. ¶ 13, J.A. 11; see J.A. 28-131. Part 8 of this disclosure listed 16 financial liabilities, identifying each creditor, and describing the nature and terms of each liability. Compl. ¶ 16, J.A. 12; see J.A. 84. The appropriate reviewing officials (see generally 5 U.S.C. app. § 106; 5 C.F.R. § 2634.605 ) determined that candidate Trumps disclosures were in apparent compliance with the disclosure requirements of the Ethics in Government Act. Compl. ¶ 13, J.A. 11; see J.A. 28.
Lovitky sees non-compliance. He alleges that then-candidate Trumps financial disclosure included both personal and business liabilities, Compl. ¶¶ 37-42, J.A. 18-19, and that this violated the Ethics in Government Act, which, Lovitky argues, requires disclosure of only those liabilities for which candidates are themselves liable ... or for which the spouse or dependent child of the candidate are liable, id . ¶ 24, J.A. 15; see also id . ¶¶ 43-44, 46, J.A. 19-20. Candidate Trump, Lovitky contends, obscured his liabilities by commingling them with the liabilities of business entities. Appellants Br. 11.
Lovitky brought suit in the district court for the District of Columbia, seeking relief in the nature of mandamus, directing [now-President Trump] to amend his financial disclosure report so that it specifically identif[ies] any debts [that then-candidate Trump] owed. Compl. 14, J.A. 21.
On April 10, 2018, the district court dismissed, holding that Lovitky lacked Article III standing because the court could not issue the relief that [he] request[ed] and thus could not redress [his] grievance. Lovitky v. Trump , 308 F.Supp.3d 250, 260 (D.D.C. 2018). Lovitky appeals, and we affirm-but on other grounds.
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We begin (and end) with subject-matter jurisdiction, without reaching the question of standing. See, e.g., Moms Against Mercury v. FDA , 483 F.3d 824, 826 (D.C. Cir. 2007) (Where both standing and subject matter jurisdiction are at issue ... a court may inquire into either and, finding it lacking, dismiss the matter without reaching the other.). In his Second Amended Complaint, Lovitky asserts three bases of federal subject-matter jurisdiction: 28 U.S.C. § 1331 (federal question); 28 U.S.C. § 1361 (mandamus); and 28 U.S.C. § 2201 (declaratory judgment). Compl. ¶ 3, J.A. 9; see also Appellants Br. 1.
But § 2201 (declaratory judgment) is not an independent source of federal jurisdiction. Metz v. BAE Sys. Tech. Solutions & Servs. Inc. , 774 F.3d 18, 25 n.8 (D.C. Cir. 2014) (quoting Ali v. Rumsfeld , 649 F.3d 762, 778 (D.C. Cir. 2011) ). And Lovitky makes no effort to challenge the district courts finding, see 308 F.Supp.3d at 260 n.8, that he abandoned reliance on § 1331 (federal question), thus forfeiting that possible font of jurisdiction for purposes of this appeal, see DeJesus v. WP Co. , 841 F.3d 527, 532 n.1 (D.C. Cir. 2016).
This leaves the Mandamus Act, which reads in full:
§ 1361. Action to compel an officer of the United States to perform his duty
The district courts shall have original jurisdiction of any action in the nature of mandamus to compel an officer or employee of the United States or an agency thereof to perform a duty owed to the plaintiff.
28 U.S.C. § 1361. Lovitky brings this action ... to compel an individual-namely, President Trump in his official capacity. See Compl. ¶ 2, J.A. 9. We assume, without deciding, that the President is an officer subject to the Mandamus Act and that the alleged obligation-to provide a full and complete statement of liabilities, see, e.g., Compl. ¶ 44, J.A. 20 (quoting 5 U.S.C. app. § 102(a)(4)); Appellants Br. 35 (same)-is one owed to the plaintiff. The only question, then, is whether the obligation sued on, see 5 U.S.C. app. § 101(c) (identifying candidates as persons required to file); id. § 102(a)(4) (identifying liabilities required to be disclosed), is a duty within the meaning of the Mandamus Act.
It is not. Lovitky has challenged only one of Trumps financial disclosure reports-the one filed in May 2016. At that time, Trumps obligation to provide a full and complete statement of liabilities arose out of his status as a candidate for the office of President, as he had not yet been elected. See 5 U.S.C. app. § 101(c) (imposing reporting obligations on candidate[s] for the office of President ... other than an incumbent President). That makes all the difference. As well develop below, duties within the meaning of the Mandamus Act include only those obligations that pertain to a defendant officers (or employees) public office. (Because Lovitky bases his claim on an obligation that was supposedly imposed by § 101(c), see, e.g., Appellants Supplemental Br. 4, we need not-and do not-express an opinion as to whether other provisions of the Ethics in Government Act impose a duty within the meaning of the Mandamus Act.)
True, if read in isolation, the phrase a duty owed to the plaintiff, 28 U.S.C. § 1361, could-at least in theory-mean any tasks that any officer of the United States happens to be obligat[ed] to perform-whether by statute, private contract, or (perhaps) even feeling[s] of moral obligation, Websters Third New International Dictionary 705 (1981).
But the context points to a narrower meaning. See, e.g., Life Techs. Corp. v. Promega Corp. , --- U.S. ----, 137 S.Ct. 734, 740, 197 L.Ed.2d 33 (2017). Here, duty is associated (in the text and title of the Act) with officer, see 28 U.S.C. § 1361 -that is, one who performs the duties of the office , Blacks Law Dictionary 977 (5th ed. 1979) (emphasis added); see also Websters Third , supra , at 1567 (defining an office as a special duty, charge, or position conferred by an exercise of governmental authority and for a public purpose). (The word officer, we note, is itself derived from the Latin word officium , meaning duty. Id .). In this context-where a statute links an officer to his duty-the most natural reading of duty refers only to those duties entailed by the office in question.
The statutes common law background also supports reading it to require that the duty a plaintiff seeks to compel via § 1361 must pertain to a defendants public office. It is a settled principle of interpretation that, absent other indication, Congress intends to incorporate the well-settled meaning of the common-law terms its uses. Sekhar v. United States , 570 U.S. 729, 732, 133 S.Ct. 2720, 186 L.Ed.2d 794 (2013) (quoting Neder v. United States , 527 U.S. 1, 23, 119 S.Ct. 1827, 144 L.Ed.2d 35 (1999) ). By using the terms action[s] in the nature of mandamus, § 1361 invokes the common-law writ of mandamus. Apart from defendants of no relevance here (i.e., common carriers, corporations, and certain public franchises, see, e.g., S.S. Merrill, Law of Mandamus §§ 25-28, at 23-28 (Chicago, T.H. Flood & Co. 1892)), the subjects of mandamus under the common law were persons who owed not just any act, but an act that appertain[ed] to their office and duty . Crocker v. Piedmont Aviation, Inc. , 49 F.3d 735, 745 (D.C. Cir. 1995) (quoting 3 William Blackstone, Commentaries *110); accord, e.g., Marbury v. Madison , 5 U.S. (1 Cranch) 137, 147, 2 L.Ed. 60 (1803) (Marshall, C.J.). The common law writ, in other words, issued to compel the performance of an act which the law enjoin[ed] as a duty resulting from an office , trust or station. Merrill, supra , § 13, at 7 (emphasis added), cited in Appellants Supplemental Br. 2 & n.3; accord, e.g., N. Pac. R.R. Co. v. Washington ex rel. Dustin , 142 U.S. 492, 506, 12 S.Ct. 283, 35 L.Ed. 1092 (1892) ; see also, e.g., Eberle v. King , 20 Okla. 49, 93 P. 748, 753 (1908) ([W]here the writ is sought to be invoked, the proper inquiry is, does the duty sought to be enforced clearly result from an office, trust, or station?); Merrill, supra , § 23, at 20 (The rule is, that this writ will not ... lie against an officer for acts done in an unofficial character.).
Detachment of the duty from the office of the defendant would lead to serious incongruities. For example, where an officer is sued in his official capacity, as here, Rule 25(d) of the Federal Rules of Civil Procedure automatically substitutes as defendant the officials successor in office. See, e.g., Nader v. Saxbe , 497 F.2d 676, 677 n.1 (D.C. Cir. 1974). Acceptance of Lovitkys theory would thus, in principle, mean that a public official could be compelled to perform the personal financial disclosure duties of his predecessor (and who knows what other duties)-an exceedingly odd result.
In sum, the Mandamus Act applies only to duties that flow from a defendants public office. And because the alleged duty here-directed at candidates for public office-lacks that defining characteristic, the district court had no jurisdiction under that Act over Lovitkys claims.
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For the foregoing reasons, the district courts judgment dismissing the case is affirmed.
So ordered.