On the Richter scale of political earthquakes, it hardly compares to Hillary Clinton's humiliating third-place finish in the Iowa caucuses on January 3, 2008 - the moment when Barack Obama's bid for the presidency truly caught fire.

But last weekend's surprisingly strong second-place finish by Bernie Sanders in the Wisconsin Democratic Convention straw poll will have sent a minor tremor through the campaign of the former First Lady as she attempts to avenge her defeat of seven years ago. Mr Sanders, the only Jewish contender for either party's nomination in next November's presidential election, polled 40 per cent to Mrs Clinton's 49 per cent.

That score should come with a huge health warning. This was a poll of largely liberal activists - Mr Sanders's core constituency - and it does not have a stellar record when it comes to predicting either party's eventual nominee. It's been wrong in each of the last four election cycles.

Nonetheless, the straw poll has finally caught the media's attention and confirmed that the Vermont senator has what Slate termed "Bernie-Mentum". At 73, Mr Sanders has thus far been seen as too old and politically exotic - he is the only self-described socialist in Congress - for the media to give his candidacy more than a cursory nod. That, combined with Mrs Clinton's supposed invulnerability, saw the New York Times relegate Mr Sanders' announcement of his candidacy to page 21, while the CBS Evening News accorded him a single sentence at the end of a two-minute report about the Democrat frontrunner.

Even before the Wisconsin result, however, Mr Sanders had begun to challenge the assumption that his campaign was safe to ignore. He has been drawing large and enthusiastic crowds in New Hampshire and Iowa - the first two states to vote next year.

Mrs Clinton's poll lead over Mr Sanders - currently close to 50 per cent - remains virtually insurmountable. However, the Vermont senator is establishing himself as her principal rival by consolidating the support of the Democrats' restive progressive base. That support had been expected to go to Martin O'Malley. But, with his attacks on inequality, calls for campaign finance reform and attempts to winkle out of Clinton her position on Mr Obama's controversial Pacific trade deal, Mr Sanders has stolen a march on the telegenic former Maryland governor.

With a series of nods to the left on criminal justice and immigration reform, same-sex marriage and workers' rights, Mrs Clinton is deftly attempting to both dampen left-wing opposition to her nomination and
fire up the Democratic core vote to turn out in next November's general election.

In the meantime, Mr Sanders's once-quixotic campaign now has the feel of an insurgency. Last week he declared: "Let me tell you a secret. We're going to win in New Hampshire." Suddenly, no one is laughing.