Within hours of North Korea's proclaimed nuclear test yesterday Dennis Hastert, the Republican speaker in Congress, and John Boehner, the Republican majority leader on Capitol Hill, issued politically charged statements. With only a month to go before mid-term congressional elections many Republicans believe the tests could help restore their waning prospects.

"This reckless move by North Korea highlights the importance of a US missile defence shield capable of protecting America against madmen with weapons of mass destruction," said Mr Boehner. "It is time for the Democrats . . . to abandon their long-standing policy of voting against missile defence programmes. It is now clear that such a position would put Americans in danger."

Republicans almost always benefit from any rise in the "fear factor" among voters in the US. However, Monday's move by Pyongyang could cut both ways. "Democrats can point to the zero progress that George Bush has made on the 'Axis of Evil' states since he identified them almost five years ago," said Bob Einhorn, who was a senior official in the Clinton administration dealing with North Korea in the late 1990s. "Iraq is a fiasco, Iran is close to getting nuclear weapons, and North Korea already has them."

On Monday the US president was more circumspect than his Republican colleagues in linking the test to the US's fetid electoral climate. In a statement condemning North Korea's "provocative action", Mr Bush hinted strongly that the US would take military action if it found evidence that Pyongyang was selling nuclear material or technology to terrorist groups or to other nation states.

Republican prospects have declined in the past two weeks following a scandal over a lawmaker's suggestive e-mails to teenage interns on Capitol Hill. The scandal, which has prompted calls for Mr Hastert's resignation as speaker, followed a revival in Republican fortunes in September that was linked to the "war on terror" after a series of speeches by Mr Bush around the fifth anniversary of the September 11 attacks.

Mr Bush's own approval ratings also rose in September in part because of public sympathy for the US president following attacks on him at the United Nations in New York by Presidents Hugo Chávez of Venezuela and Mahmoud Ahmadi-Nejad of Iran. There was also a rising focus on the Democratic party's divisions over the war in Iraq, with some party leaders calling for immediate withdrawal and others for a better military strategy on the ground.

Monday's tests could help the Republicans regain control of the news agenda. "Our message has been blown off course in the last few weeks," said Saul Anuzis, head of the Republicans in Michigan, one of the key battleground states. "To the extent this focuses people's minds on the dangers America faces it will help Republicans."

However, North Korea inhabits a far smaller space in the American mindset than Iran, in part because – unlike Tehran – Pyongyang poses no direct threat to Israel or to the increasingly combustible situation in Iraq. In addition, the Bush administration has come under attack from moderate Republicans for its reluctance to hold direct talks with either Iran or North Korea.

"The best thing both parties could do is issue a joint resolution condemning North Korea," said Michael Green, a former senior Bush official now at the Centre for Strategic International Studies. "North Korea misinterprets domestic criticism of Mr Bush as weakness. They do not understand our political system."