Sen. Barack Obama, hoping to widen his party's control of Congress, has started to lend support to down-ticket Democrats.

The approach marks a shift in strategy for the Democratic presidential candidate, who until recently has shunned joint appearances with other candidates. Party congressional strategists said the distancing had frustrated some Democrats, but that Sen. Obama had felt such appearances could dilute his brand and diminish his outsider appeal.

Now, with his lead in the polls holding steady, the Illinois senator has begun to volunteer his campaign staff to assist fellow Democrats, channel funds to state Democratic parties and appear himself on the campaign trail with other candidates.

Tuesday evening, under a darkening Miami sky, Sen. Obama stood on a stage with three Democratic candidates hoping to unseat entrenched House Republicans from their South Florida districts. The next day, he twice appeared with former Virginia Gov. Mark Warner, who is running for a Senate seat, and concluded a swing through the Old Dominion with a nod to Democrat Judy Feder and her long-shot campaign against Republican Rep. Frank R. Wolf in Northern Virginia's 10th district.

Obama campaign officials are still hesitant to acknowledge the efforts, fearing anything that could make them look overconfident. They also worry that producing an advertisement or holding a fund-raiser for one House candidate could trigger an avalanche of requests. And they don't want their candidate to lose sight of his real goal, the presidency.