By Ernest Scheyder and Joseph Ax
HOUSTON, Sept 13 (Reuters) - The Democratic presidential contenders head back to the campaign trail on Friday after a debate that reinforced former Vice President Joe Biden's front-runner status and left his rivals searching for a formula to knock him out of the top spot.
The candidates who participated in Thursday's debate in Houston, and those scrambling to try to qualify for next month's debate in Ohio, have less than five months to plead their case to voters before the first nominating contest in Iowa on Feb. 3, 2020.
In the Houston debate, Biden clashed with progressive rivals Elizabeth Warren and Bernie Sanders on healthcare, challenging the two U.S. senators to be honest about the cost of their plans.
But the top 10 Democratic candidates, sharing the debate stage for the first time after the party toughened the rules to qualify, stressed their shared opposition to Republican President Donald Trump more often and pared back some of the bickering that marked the first two debates this summer.
Biden was sharper and more aggressive than in the earlier matchups, leaving him in a strong position as the pace of the campaign picks up and his rivals evaluate the best strategy in the race to pick a Democratic challenger to Trump in the November 2020 election.
"I'm going to continue to talk about why we need to unify the country around our common goals and our aspirations and our common problems," U.S. Senator Kamala Harris of California, who refrained from attacking her rivals during the debate, told reporters afterward.
Several major events are on the campaign schedule in coming weeks. Biden, Sanders and at least three other candidates are scheduled on Monday to attend the Galivants Ferry Stump meeting in South Carolina, a state party tradition dating back to 1876.
The next weekend, 18 candidates will attend the Polk County, Iowa, steak fry, a regular stop for presidential contenders, and at least 10 candidates will participate in a forum on LGBTQ issues.
After that, Democrats plan up to nine more debates, beginning next month in Ohio.
At least one more candidate, billionaire activist Tom Steyer, has met the heightened qualifications for the next debate, meaning it is likely to be spread over two nights as the first two were in June and July.
Candidates who do not qualify for that debate, and those on stage on Thursday who are struggling in the polls, will have to decide whether they have a reason to keep running. Aside from Biden, Sanders and Warren, none of the other 17 Democratic contenders is regularly hitting double digits in support in opinion polls.