For most European leaders the treaty is a big step toward a European Union that is more effective, more accountable and better prepared to take on big issues on the world stage.
Not for vehemently Euro-skeptic Klaus, who has been tirelessly attacking the treaty _ and now finds himself in the unique position of potentially blocking it.
Klaus is a largely ceremonial head of state _ like the Queen of England _ and since both houses of Parliament have ratified the treaty, he is all but legally bound to sign the document.
However, speculation runs high he may try to hold off on his blessing until British elections next year _ which the famously Euro-skeptic Conservatives are expected to win easily.
That could prove fatal for the treaty: The Tories have pledged a referendum on the charter if it has not been ratified by the time they are elected. With the British traditionally aloof _ if not downright hostile _ toward continental Europe, they would be expected to vote no.
The reason Klaus may have some stalling time is that a group of anti-EU Czech senators have mounted a challenge to the treaty in the nation's Constitutional Court.
The tribunal is expected to reject their charges of unconstitutionality (it has already rejected a similar challenge in the past) but if the proceedings drag on, Klaus may just have a chance of withholding his signature until the crucial British poll.
Failure of the Lisbon treaty would send the EU into an unprecedented crisis.
At stake for Europe are lofty plans for new posts of EU president and foreign minister, a streamlined decision-making process, and more powers to the European Parliament. Treaty negotiators say the reforms are needed to make the EU function more effectively in line with its rapid growth eastward since 2004.
The conservative Klaus relishes being at odds with the mainstream. He is outspoken in questioning the widely accepted view that humanity is the probable cause of global warming, and is a strong opponent of gay marriage in what is considered one of Europe's more liberal nations.
"The fate of the Lisbon Treaty, a document for a half billion community, is entirely in the hands of the Czech president," the Czech daily Lidove Noviny said in a recent editorial. "Will he sign, in the end? Or will he not?
"A cautious person would not bet on it."
Klaus argues that the treaty gives too much power to Brussels and strips individual countries of their sovereignty by removing their veto power and replacing it with majority voting.
"The Lisbon Treaty is a step back to the rejected European Constitution," Klaus said recently on his official Web site. "It is a document that represents a significant shift from an Europe of states to an Europe of one single European state."
Klaus remains unflappable amid heated speculation about his intentions.
"Consider the (senators') criticism serious and I expect the Constitutional Court will provide convincing answers," Klaus said last month. "It is only on the basis (of those answers) that I can consider further action."
But he makes no secret of his stance.
"My views are known and clear," he said after the upper house approved the charter in May. "I cannot afford to be, at one moment, absolutely against (it), and then, just easily change my mind just because it is about to suit my political or career goals."
Those close to him suggest that his comments are vintage Klaus. Ladislav Jakl, the president's chief political adviser, said it would be typical for Klaus not to decide until "he has to."
Among factors that will tip his decision, Jakl said, would be the Constitutional Court ruling nd the public's reaction to that decision.
Even before the court weighs in, however, it seems clear most Czechs want the treaty.
A Sanep agency poll showed 53 percent of 4,320 respondents aged 18-66 saying they would approve it in a referendum, with 44 percent thinking Klaus is damaging the country's image by withholding his signature and 43 wanting him to sign without delay.
The margin of error for the survey conducted Oct 1-3 was plus or minus 2.5 percentage points.
"In the end, President Klaus will sign the treaty," said European Commission President Jose Manuel Barroso last week, while French President Nicolas Sarkozy warned of unspecified "consequences" for the Czech Republic if Klaus continues to withhold his signature.
But Klaus seems unimpressed _ both by the polls and a full-court press in Brussels and elsewhere meant to budge the contrarian president.
Instead of buckling to pressure, Klaus is making demands _ calling for Czech exemption to a human rights charter included in the treaty that the Poles and the British have also secured.
On the home front, pro-treaty politicians are girding for battle, if Klaus withholds his signature despite Constitutional Court rejection of objections to it.
"If he rejects, someone else has to do it," said deputy Senate speaker Alena Gajduskova.
She said that both parliamentary chambers could vote to strip the president of his power to approve the treaty if he fails to act and instead have Prime Minister Jan Fischer sign.
Klaus could even be charged with high treason, Gajduskova said, while adding that was unlikely.
Jahn reported from Vienna. Associated Press writers Ondrej Hejma in Prague and David Stringer in London contributed to the report.