U.S. Senator Tim Johnson’s decision to retire after the 2014 election makes the Democrats’ job of holding their Senate majority tougher.

Johnson, 66, the Senate banking committee chairman, is the second Democrat from a state won by Republican nominee Mitt Romney last year to say he won’t seek re-election in 2014. His announcement yesterday means that in Republican-leaning states, his party must defend those two open seats and support five incumbents running for re-election.

His announcement sets up a possible fight over who would head the Senate Banking, Housing & Urban Affairs Committee — which oversees financial regulation legislation — in the 2015- 16 congressional session.

In all, 21 Senate seats now held by Democrats are up for re-election next year, and five of them have been opened up by retirement announcements. Among Republicans, 14 Senate seats will be on the ballot and only one of the party’s incumbents — three-term Senator Susan Collins of Maine — is seeking re- election in a state Romney lost to President Barack Obama. So far, Republicans must defend two open seats, in Georgia and Nebraska, both of which Romney carried.

“The battle for the Senate will come down to Democrats’ ability to hold seats in Republican-leaning states,” said Nathan Gonzales, deputy editor of the non-partisan Rothenberg Political Report, based in Washington. “A lot depends on how popular the president is” next year “because if people are dissatisfied, their option is to vote against the president’s party.”