WhenSen. Marco Rubio (R-Fla.) decided to run for re-election, he moved the needle in favor of the Republicans keeping their Senate majority. Now with former Indiana Governor and Senator Evan Bayh’s decision to run again for the Senate the needle moves back in Democrats favor. Rep. Baron Hill (D-Ind.) agreed to clear the field for Bayh.

Recall that Democrats need 5 seats for an outright majority, 4 for a tie if Hillary Clinton wins. The GOP is likely to pick up at most one state (Nevada). Illinois and Wisconsin are the most likely pick-ups for Democrats with Ohio, New Hampshire and Pennsylvania all toss-up’s. With Florida back in the “R” column, a win for the Democrats in Indiana means a Clinton win would require the Republicans to sweep the three toss-up states and win Nevada to keep the Senate majority (assuming Illinois and Wisconsin are lost).

The good news for Republicans is that there nominee has a fighting chance to win with Rep. Todd C. Young, a mainstream Republican and Marine veteran. As the Post reported in May, “Allies of Senate Majority Leader Mitch McConnell (R-Ky.) poured millions of dollars into backing Young [in the primary] to prove that mainstream Republicans can retain control of Congress in the year of Donald Trump.”

Unlike Illinois, New Hampshire and Wisconsin, the good news for Republicans is that Indiana has voted Republican for president in every presidential election dating back to 1976, with the exception of 2008 when President Obama won by one percentage point. Donald Trump therefore will be less of a drag on the down-ticket Republicans in Indiana than he will in blue and purple states.

That said, Bayh’s decision cannot be considered anything but a serious blow to Senate Republicans. The good news for the country and the Senate however may be that Bayh is one of a dying breed of moderate, hawkish Democrats. If Clinton wins the White House and Bayh wins the Indiana Senate seat, the moderate wing of the Democratic Party would make a bit of a comeback, in stark contrast to the growing prominence of the party’s leftwing in recent years.

Speculation is swirling that current Governor Mike Pence may be on Trump’s list for vice president. It is hard to imagine however that Pence, up for re-election as governor, would risk his political future and credibility with Trump, whose views on entitlements, foreign policy and other key issues are significantly to Pence’s left. Why put at risk the governorship for the slim chance of becoming vice president, which could well leave Democrats with the governorship and both Senate seats?

The Bayh announcement highlights the GOP’s serious dilemma. If Trump walks out of Cleveland as the party’s nominee the GOP will have succeeded in putting up the one Republican contender capable of losing to Clinton and thereby lowering the barrier to the Democrats’ opportunity to win the Senate (from 5 to 4 seats). What would ordinarily be a safe red state — Indiana — becomes one more competitive battleground in which Democrats could find a path to seizing the White House and the Senate.