By Bo Erickson and Nandita Bose

CHILLICOTHE, Ohio/MORROW, Georgia, Oct 28 (Reuters) - Neither man is on the ballot but Republican President Donald Trump and Democratic former President Barack Obama took prominent roles on the campaign trail on Wednesday, with less than a week to go before congressional races that could shift the balance of power in Washington.

While Trump has been on a weeks-long campaign blitz that ‌has underlined Republican concerns about losing control of Congress for the final two years of his presidency, Obama had kept a relatively low public profile until the past week, during which he has delivered blistering ‌speeches criticizing Trump.

Thirty-five of the 100 Senate seats and all 435 seats in the House of Representatives are being contested next Tuesday. Nonpartisan analysts generally favor Democrats' chances of capturing a majority in the House, with the contest for the Se...