TAMPA, Florida (Reuters) - Republicans formally nominated Mitt Romney as their presidential candidate on Tuesday to face Democratic President Barack Obama in the November 6 election.

By voice vote in a raucous arena, delegates to the Republican National Convention united behind the former Massachusetts governor, who this year outlasted a series of conservative challengers to come out on top.

Romney, 65, is in a close fight with Obama with 70 days to go until the election and Republicans hope he will gain some much-needed momentum from the Tampa convention this week.

(Reporting by Steve Holland; Editing by Jim Loney)