WASHINGTON — The ghost of Howard H. Baker Jr., the Republican senator from Tennessee who turned against Richard M. Nixon during Watergate, is hovering over Senator Lamar Alexander.

Mr. Alexander, a third-term Republican from Tennessee who is retiring at the end of this year, has said that no one outside his family has had more influence on him than Mr. Baker, the former Senate majority leader who is remembered for the penetrating question he posed as Nixon stared down impeachment: “What did the president know, and when did he know it?”

Now Mr. Alexander may hold in his hands the fate of another Republican president who is facing removal from office. He is one of four Republican moderates who have expressed openness to bringing witnesses into President Trump’s impeachment trial. Of the four, he stands out because he is not running for re-election and arguably has nothing to lose.

Yet as the Senate heads toward a vote on the matter, Mr. Alexander — who has broken with Mr. Trump over trade, the border wall and health care — does not appear ready for a Howard Baker moment. He has said he will make a decision about witnesses after Mr. Trump’s team presents its defense and senators have an opportunity to ask questions, but he does not sound eager to defect.