Democrat Haley Stevens has endorsed Republican Mike Rogers in Michigan’s tightly contested Senate race, backing Abdul El-Sayed’s rival as her own party seeks to overturn Republican control of Congress.

The cross-party endorsement comes as campaigning reaches its final day before the US midterm elections. Donald Trump is rallying support for Republicans, while Democrats make their closing arguments in contests that will determine control of Congress for the remainder of his presidency.

Stevens’s decision puts her at odds with her party’s electoral objective in Michigan. It gives Rogers support from across the party divide, but does not establish whether Democratic voters will follow her lead. Its effect on the close Senate contest remains uncertain.

Republicans currently hold both the Senate and the House of Representatives. Democrats hope to gain enough seats to secure a majority, entering the elections with high expectations. Whether the polls accurately reflect the eventual vote remains an important uncertainty.

All 435 House seats and 35 of the Senate’s 100 seats are being contested. Voters will also choose politicians at state level, including governors.

Millions of Americans have already cast their ballots. In Georgia, a third of registered voters have voted early, setting a new midterm record in the state. That establishes the scale of early participation, not which party stands to benefit.