Spain's opposition Socialists said today they would call a symbolic vote of no-confidence against Prime Minister Mariano Rajoy if he refused to appear before parliament to answer questions about a deepening scandal over party financing.
Mr Rajoy's ruling centre-right People's Party has an absolute majority in parliament.
Unless there were significant defections from members of his party, he would survive the vote.
But a motion of no-confidence, which has been used only twice since the death of dictator General Francisco Franco in 1975, would involve him or a representative appearing in parliament to defend his actions.
Until recently Mr Rajoy had managed to limit the impact of the scandal, which involves alleged illegal donations by construction magnates that were supposedly distributed as cash payments to party leaders in return for contracts.
Today he told business leaders at a lunch that he would continue to reject calls to resign and his strong majority in parliament was a guarantee of political stability in Spain, according to a source who was briefed on the meeting.
Mr Rajoy, who came close to having to ask for a financial rescue last year when the eurozone crisis was at its worst point, is at pains to differentiate his leadership from less stable coalition governments elsewhere in southern Europe.
Facing growing pressure within the PP over his handling of the corruption scandal, he said he would stick to his political reform programme until the 2015 election.
But pressure from other political groups is mounting.
"I know I don't have enough seats [to win a no-confidence vote] but I want him to appear before this chamber," Socialist leader Alfredo Perez Rubalcaba said in a televised speech to lawmakers from his party.
In a meeting of the standing committee of parliament this afternoon, the PP rejected new opposition calls for Mr Rajoy to appear for questioning in the Senate.
The committee is due to discuss on 24 July Mr Rajoy's possible appearance before the lower house. Depending on the results of the talks, the Socialists will then move forward with their threat.
At the heart of the affair is former party treasurer Luis Barcenas, 55, who was arrested in June and charged with bribery, money laundering, tax fraud and other crimes.
He says he had made €90,000 in cash payments to Mr Rajoy and party Secretary General Maria Dolores Cospedal in 2009 and 2010 - an allegation Mr Rajoy denies.
If the former treasurer did make such cash payments, they would not necessarily be illegal income if the recipients declared it to tax authorities.
Mr Barcenas has hinted he has evidence that could implicate Mr Rajoy and others in crimes. Yesterday, Mr Rajoy said he would not give in to "blackmail".