TAMPA — Has there ever been a more dishonest presidential campaign than the one Republicans are waging right now?

That’s a serious question, and it adds poignancy to the tragicomic spectacle of this frankly ridiculous gathering. The one indisputable truth we hear from speaker after speaker at the Republican National Convention is that this is a consequential election. The country faces huge challenges and fundamental choices, and the two major parties have very different ideas about the way forward.

Anyone familiar with this column knows that I prefer the progressive vision over the conservative one. But I believe it’s not possible for the nation to set a course without a vigorous, honest debate — and I know there can be no such contest of ideas without agreement on factual truth.

Vice presidential candidate Paul Ryan’s speech Wednesday night was another demonstration that he and presidential nominee Mitt Romney have no apparent respect for the truth. Romney’s pollster, Neil Newhouse, boasted this week that “we’re not going to let our campaign be dictated by fact-checkers.” I’ll say.

Ryan built his career on a reputation for wonkish immersion in the details and willingness to tell uncomfortable truths. But in his address to the convention he lied and dissembled so shamelessly that I thought I detected a whiff of desperation in the air. Or maybe it was just ambition.

The whopper with which those pesky fact-checkers are having a field day is Ryan’s attempt to blame President Obama for the shutdown of a huge General Motors plant in Ryan’s hometown of Janesville, Wis. Ryan’s point of reference was a visit Obama made to the plant during the 2008 campaign.

“A lot of guys I went to high school with worked at that GM plant,” Ryan said. “Right there at that plant, candidate Obama said: ‘I believe that if our government is there to support you . .. this plant will be here for another hundred years.’ That’s what he said in 2008. Well, as it turned out, that plant didn’t last another year. It is locked up and empty to this day. And that’s how it is in so many towns today, where the recovery that was promised is nowhere in sight.”

In other words, Obama promised to help those workers by keeping the plant open but failed to deliver. This is a bald-faced lie.

As Glenn Kessler, author of The Washington Post’s Fact-Checker column, has noted, Obama visited the Janesville plant in February of 2008. GM announced the plant’s shutdown in June 2008 — five months before Obama was elected and seven months before he took office. Ryan should be blaming George W. Bush, not Barack Obama.

And technically, the plant isn’t even closed. It’s on “standby,” according to GM, and can be reactivated if the demand for production rises sufficiently.

Ryan was careful with his words. He didn’t specify who was president when the plant was ordered to cease production. He described it as “locked up and empty,” rather than “closed.” But by any reasonable standard, Ryan was being deceptive. He wanted his listeners to believe something that simply is not true.

Another supremely dishonest moment was Ryan’s criticism of how Obama dealt with the Simpson-Bowles debt panel: “He created a bipartisan debt commission. They came back with an urgent report. He thanked them, sent them on their way, and then did exactly nothing.”

Lordy. Ryan failed to mention that he was a member of the Simpson-Bowles commission. He also failed to mention that he was part of a minority of panel members who flatly rejected the “urgent report” he now blasts Obama for ignoring. That report, you see, endorsed a balanced approach including not only budget cuts but also revenue increases. Can’t have that.

To be sure, these are not the worst of the Republican lies. For me, the ultimate dishonor goes to the untrue charge that Obama has eliminated the work requirement for welfare recipients — a lie designed not only to deceive but to stoke racial resentment among working-class voters. There are also the cynical and misleading claims about Medicare savings under the Affordable Care Act. And Obama’s “You didn’t build that” line, taken out of context, has provided the convention’s main theme, a mantra recited by virtually every speaker.

It’s hardly unusual for politicians to highlight convenient facts and ignore inconvenient ones. But I honestly can’t recall a campaign so firmly grounded in untruth. Anyone who hoped Ryan might elevate the debate should be bitterly disappointed.