As former President Donald Trump’s second impeachment trial began Tuesday, all eyes were on one set of jurors: Republican senators who are up for reelection next year.

Republicans need a net gain of only one seat to win back the Senate, now split 50-50 and under Democratic control because of Vice President Kamala Harris’s tiebreaking vote. But they are also defending 20 seats to the Democrats’ 14, with GOP-held seats in Ohio, Pennsylvania, and now Alabama opening up due to retiring incumbents. Republicans will try to hold seats in two states won by President Biden, while the Democrats are defending none in states Trump won last year.