DENVER » In a bitter fight, Colorado Democrats recently muscled through the Statehouse a massive elections reform bill that allows voters to register up until Election Day and still cast their ballots.

It’s the latest — and most substantial — development in a nationwide Democratic Party effort to strike back at two years of Republican success in passing measures to require identification at polling places and purge rolls of suspect voters.

Democratic-controlled states like California, Connecticut and Maryland also all have sought to make it easier to cast a ballot as late as possible. They recently passed versions of same-day voter registration measures, which traditionally help younger and poorer voters — the sort who lean Democratic.

Undaunted, the GOP is aggressively fighting the efforts.

Maine Republicans tried to roll back same-day registration in 2011 but were unsuccessful. And Montana Republicans hope to rescind their state’s same-day registration through a ballot referendum next year.

In the decades-old battle between Republicans and Democrats over voting rights, same-day voter registration long had been a relatively bipartisan matter, a staple in places like Idaho, New Hampshire and Wyoming. But it has become a divisive issue in recent years, as the country has grown more polarized and as both major political parties seek to change voting laws in ways that will be beneficial to them.

“There’s been more partisanship over the last half-decade as the voting wars heated up and as groups spent more energy on process and the nuts and bolts of how elections are run,” says Rick Hasen, a law professor at the University of California, Irvine. “Colorado is ground zero right now for these battles.”

Colorado is the 11th state to allow same-day registration, according to the National Conference of State Legislatures.

Compared with previous years, Myrna Perez at the Brennan Center for Social Justice at New York University said that in the aftermath of the 2012 elections, “We saw more positive expansive legislation being introduced in this legislative cycle across the country than we saw restrictive.”

According to the Brennan Center, 204 bills to expand voting in 45 states were introduced this year, with many measures still pending. Among the new laws are ones that allow online voter registration in Virginia and West Virginia, ease photo ID restrictions in Oklahoma, and expand early voting in Maryland.

By comparison, 31 states introduced 82 bills that sought to restrict voting access, with 50 bills still pending. In Arkansas, lawmakers overrode a gubernatorial veto to require photo ID to vote. Nebraska reduced the early voting period. And Montana is trying to repeal Election Day registration with a ballot question in 2014.

Republicans in that state are seeking to roll back the state’s 7-year-old same-day registration law. Democrats tried to shut down the state Senate to block the measure. It didn’t work, and the question goes before the voters next year.

“They don’t want to make it easier for college students to register to vote and for Native Americans to register to vote,” Democratic state Sen, Kendall Van Dyk said of Republicans.

Republican State Sen. Alan Olson countered that the proposal would just end registration the Friday before Election Day in order to make life easier for clerks in small rural counties swamped by last-minute registrants. He said that’s still a vast improvement over the prior registration deadline of 30 days before the election.

In California, former state Assemblyman Mike Feuer said the spread of voter identification measures in GOP-run statehouses in Florida, Pennsylvania, Wisconsin and elsewhere inspired him to write his state’s same-day registration law. “At precisely the moment when other states were moving to suppress voter turnout, it was important for California to say: ‘We can do better,'” Feuer said.

Well before it passed the new law, Colorado had been at the forefront of the national debate over voter access because of Republican Secretary of State Scott Gessler’s history of fighting Democrats over a variety of election issues, including efforts to weed out suspected noncitizens from the rolls.

But Democrats were emboldened in the aftermath of a highly successful November election season. It saw Obama win the state, and, more consequential, the party winning back the state House and keeping power in the state Senate. With a Democratic governor at the helm, the sweeping victories gave the party wide leeway to pass the bills they wanted.

Democrats quickly took advantage of their power gains, passing measures to help people in the country illegally, expand firearm background checks and allow same-sex civil unions. Then, late in the legislative session, they introduced their elections overhaul bill that allows same-day registration. It also requires that every voter get a mail ballot.

A bipartisan coalition of local county clerks backed it, saying it would increase access to the ballot box. But Gessler, the Republican who is the state’s chief elections official, repeatedly said he was purposely excluded from the process — a charge Democratic lawmakers denied. And legislative Republicans were horrified by the 128-page bill, and warned of fraud that could accompany same-day registration.

“You’re already winning the elections,” GOP State Senate leader Bill Cadman said as the GOP tried to block the legislation. “Do you need to steal them, too?”

Democratic Gov. John Hickenlooper signed the measure on May 10.