Californians will be able to register to vote on election day at local polling places and voting centers under legislation signed by Gov. Gavin Newsom on Tuesday, a potentially significant step toward boosting turnout in key contests next year. The new law provides for a significant expansion of so-called conditional voter registration, which allows a new voter to cast a ballot that is counted after eligibility is determined during the 30-day vote-counting period after an election. That process began in last year’s statewide election, but registration was available only in county elections offices. Starting next year, voters can register on election day anywhere ballots are cast.