Perhaps the most curious thing about the six-day window that allowed thousands of Ohioans to both register to vote and cast early ballots on the same day was that Republicans were so blind-sided by it.

After all, as is often noted, GOP state lawmakers were the architects of the bill allowing the early voting that caused all the crowds. Why would the party that wrote and passed a law champion legal action against it, as Republicans did — albeit unsuccessfully — this year?

The 2005 law allows anyone to vote early using an absentee ballot, by mail or in person, without stating a reason, referred to as no-fault absentee voting. This is the first presidential election in which the practice has been in place.

A short overlap period exists between the date absentee voting begins and the date voter registration ends, allowing the same-day registration and voting, but those close to negotiations on the original law say the scenario simply never came up.

‘‘It was never discussed, not once,’’ said Catherine Turcer, who observed the process for the government watchdog group Ohio Citizen Action.

‘‘Nobody ever worried about it. It was just there,’’ said Peg Rosenfield, an election expert with the Ohio League of Women Voters. ‘‘I knew it was in the law. You’ve been able to register and cast an absentee vote in this state on the same day since at least 1981.’’

In 2005, the state was recovering from an intense election. Ohio had just helped deliver the White House to Republican President Bush for a second term and a coalition of Democrat-backed groups was pushing a package of sweeping government reforms to Ohio’s Constitution.

The Reform Ohio Now, or RON, amendments would have revamped Ohio’s redistricting process, imposed new restrictions on campaign donations, stripped then-Secretary of State Ken Blackwell and his successors of much authority and legalized the no-fault absentee voting.

Republicans in the Legislature, and many Democrats, worried that the amendments were sloppily worded and needed more debate.

Inserting the no-fault absentee voting issue, arguably the one most popular among voters, into another voting-related bill was among their strategies for defeating the measures. (It worked — all four issues failed.) The bill, sponsored by Republican Rep. Larry Wolpert of Hilliard, was initially intended to address the state’s poll worker shortage by allowing 17-year-old high school seniors to work on Election Day.

Debate began shortly thereafter on House Bill 3, a massive rewrite of Ohio election law championed by state Rep. Kevin DeWine of Fairborn, who now serves as the deputy chairman and mouthpiece for the Ohio Republican Party.

The bill contained a controversial and much-publicized requirement that Ohio voters bring identification to the polls. From start to finish, it took a year to craft, negotiate and enact — finally taking effect in May 2006.

Amid all the hubbub over that bill, lawmakers left untouched the portion of state election law containing the overlapping absentee and registration deadlines — and creating a window that Democratic presidential candidate Barack Obama appears to have deftly exploited this year.

Because of the way the two no-fault absentee provisions came to be, no research or debate took place about what effectively became full-scale early voting in Ohio last week, Rosenfield said. Thousands of voters around the state visited government-run polling places and one-stop shops set up at college campuses and other locations by the Obama campaign.

Ironically, Rosenfield could have provided many of the answers state lawmakers needed three years ago. She is familiar with early voting and vote-by-mail laws around the nation, having authored the Federal Election Commission’s publications on them.

Nobody asked.

‘‘This was explaining if you’re going to do this, this is how they do it in all these other states,’’ she said. ‘‘Before I wrote the thing, I spent a week in Washington state in Olympia watching how they did an election by mail, I talked to people in the Secretary of State’s office, in the county election offices, and I watched them do it.’’