COUNCIL BLUFFS, Iowa—In less than a month, Mayor Pete will just be Pete.

Pete Buttigieg’s transition on Jan. 1 to ordinary citizen from mayor of a northern Indiana city of about 100,000 people, after he didn’t run for re-election this year, could bring added attention to what some of his rivals have argued are modest credentials for election to the highest office in the land.

But there may be an upside for Mr. Buttigieg. While he has mostly been an on-the-road mayor of South Bend, Ind., this year, his exit from office will free up his schedule even more, which could come just as some of his rivals for the Democratic presidential nomination are forced back to Washington to hear evidence in a possible impeachment trial of President Trump.

If a trial does take place, Mr. Buttigieg and other candidates who aren’t in the U.S. Senate, such as former Vice President Joe Biden, may have the campaign trail much more to themselves ahead of the Feb. 3 Iowa caucuses that start the nominating process. Mr. Buttigieg has led in recent polls in the state and is looking to sustain that momentum.

“The impeachment trial will provide an opportunity for the senators running for president to lay out a substantive Trump critique that will reach some portion of the electorate,” said Ben LaBolt, a longtime adviser to former President Obama. “But this may present even more of an opportunity to the candidates that are free to spend time on the ground engaging with voters in the states and media markets they are targeting with fewer opponents competing for space.”