President Reagan has decided to nominate Jane Richards Roth, the wife of Sen. William V. Roth Jr. (R-Del.), as a federal judge after a period of maneuvering in which the senator suggested her name to then-Attorney General William French Smith.

The Justice Department is conducting a preliminary background check on Jane Roth, 49, an attorney in a Wilmington law firm for 20 years. The Harvard Law School graduate is expected to be formally nominated soon to the U.S. District Court in Wilmington.

Roth is a partner in Richards, Layton & Finger, an old-line firm founded by her grandfather that represents a number of Delaware banks and insurance companies. Her father, a former Republican national committeeman, was a senior partner in the firm, which now includes her brother and cousin. Roth, who is highly regarded by Delaware attorneys, has handled a range of civil cases and specializes in defending medical malpractice suits.

As Delaware's senior senator, Roth has successfully recommended several lawyers for federal district judgeships. The president generally follows a longstanding tradition of accepting such recommendations from the senior senator of his party in each state.

Roth acknowledged that it is "somewhat awkward" for him to be recommending his wife. Nevertheless, he said, "Whenever anyone asks, I say I strongly support her."

Roth's wife initially promoted herself with Justice Department officials for a seat on the 3rd U.S. Circuit Court of Appeals, but was passed over in favor of the chief district court judge in Wilmington. The senator said he then asked Smith to consider her for the district court vacancy.

"I said that if they were looking for women, I suggest they look at my wife," Roth said.

An administration official said Reagan called Roth a few weeks ago to make sure the appointment would not create political difficulties for him. Roth said the president told him that "he was very interested in nominating her and he would appreciate if I would defer to him."

The three-term senator, who is also a lawyer, said reaction in Delaware has been positive because "there's a general recognition in the state that she's preeminently qualified."

Delaware Bar Association President Francis Biondi said Roth is a well-regarded trial lawyer and "is next to the most senior woman practicing at the bar in the state." Democratic state chairman Sam Shipley called her "a highly qualified and well-respected person."

But Jeffrey Hazard, a Yale Law School professor and expert on legal ethics, said Roth's actions involve "the quintessence of the appearance of impropriety." He said it could be argued that Roth is violating the American Bar Association's ethics code.

Hazard said it is not clear whether the federal anti-nepotism law would apply to a recommendation or vote by a member of Congress. The law prohibits a public official from appointing or advocating a relative for a job over which he has "jurisdiction or control."

While it is not unusual for senators to recommend political supporters, former law partners or fellow lawmakers as federal judges, no one could recall a case in which a senator supported his wife. Many senators rely on judicial screening panels, but Roth said he informally consults with an "ad hoc group" in recommending federal judges.

Roth said he planned to introduce her to the Senate Judiciary Committee at her confirmation hearing, but has not thought about whether he should abstain from voting for her on the Senate floor.

The administration selected Jane Roth after she was interviewed by White House counsel Fred F. Fielding and top Justice Department officials. An administration official said the senator has been "very careful" not to pressure anyone and is "sensitive to the charge that he's promoting his wife."

Jane Roth said yesterday that she has unsuccessfully sought to become a state judge, but twice was passed over by a Delaware screening panel. Asked about her husband's support, she said: "I believe if I were not qualified, his recommendation would make no difference at all.

"I think, frankly, the fact that I'm a woman and the administration is looking for qualified judges who are women . . . has been an advantage," she said. The 165 federal judges that Reagan named in his first term included 13 women and two blacks.

Roth said she told Justice Department officials on several occasions that she wanted to be a judge. She said she made her case to Smith at a diplomatic breakfast in London last year while accompanying her husband on Senate business.