By Jeremy McDermott BBC Correspondent in Medellin

Colombia's Marxist guerrillas have been conducting their own political campaign for Sunday's congressional elections.

It has involved kidnapping and murdering politicians and calling for a national boycott; and it has been very persuasive.

Kidnapped

Presidential candidate Ingrid Betancur Senator Jorge Gechem Turbay

Since the government broke off peace talks last month and invaded the safe haven of the Revolutionary Armed Forces of Colombia (FARC), the guerrillas have launched a nationwide campaign against infrastructure, the cities and now politicians.

The event that triggered the breakdown of the peace process last month was the meticulously planned hijacking of a domestic flight with a well-known senator on board.

Rebel roadblock

The plane was forced to land on a road near the now-defunct guerrilla safe haven and Senator Jorge Gechem Turbay joined five other politicians in captivity.

He was soon joined by the presidential candidate Ingrid Betancur, caught in a rebel roadblock in the same area.

These abductions are designed to pressure the government into exchanging the hostage politicians for imprisoned rebels.

Killed

Ex-Culture Minister Consuelo Araujo Representative Diego Turbay Senator Martha Catalina Daniels

But the attacks on politicians have not been restricted simply to kidnappings.

A cousin of the kidnapped Senator Turbay, Representative Diego Turbay, was dragged from his vehicle in December 2000 and shot alongside his mother and three bodyguards.

In September last year, FARC kidnapped the popular former Culture Minister Consuelo Araujo.

When security forces seemed about to catch up with the fleeing rebel column, she was forced onto her knees and shot five times in the back of the head.

Canvassing by video

And if the Prosecutor General office is to be believed, FARC was also responsible for the execution of Senator Martha Catalina Daniels.

She was found in her car with two shots to the head earlier this week, although FARC has denied involvement.

February's plane hijack sparked the latest crisis

This has meant that many candidates for the elections have not gone out, particularly into the rebel dominated rural areas, to canvass voters.

Last week's re-election campaign rally by Senator Claudia Blum in the eastern city of Bucaramanga was a perfect example.

Her anti-corruption manifesto was laid out before enthusiastic supporters who cheered and pledged support - to a video screen from where the address came, as Senator Blum was safely tucked up in her heavily guarded flat in Bogota.

"I was told that security forces had intercepted a radio conversation in which Mono Jojoy [the alias of the FARC field marshal] gave instructions to go after [candidates], and my name was mentioned explicitly," Senator Blum said.

None of the candidates... will legislate in favour of the people and those of them who seek to will be assassinated

FARC statement

The 17,000-strong FARC organisation has called on Colombians not to vote in Sunday's elections from which 102 senators and 163 representatives will be picked.

"None of the candidates for the House of Representatives or Senate in the whole country will legislate in favour of the people, and those of them who seek to will be assassinated as has been shown in the recent history of the Patriotic Union," read a FARC statement.

The Patriotic Union was a party set up in the 1980s under FARC auspices and which had 3,000 of its members killed by right wing paramilitaries and drugs traffickers.

There has been some weight added to the guerrilla discrediting of the candidates, albeit from an unexpected quarter.

The prosecutor-general's office said on Friday that at least 100 candidates were under investigation and that more than 50 had criminal convictions - some for extortion and rape - which made them ineligible to take their seats should they be elected.

Paramilitary enemies

But it is not believed that FARC is planning widespread disruption of the elections - certainly not in the 40% of the country it dominates where it can influence who is voted for.

FARC's right-wing paramilitary enemies in the United Self Defence Forces of Colombia (AUC) are doing the same thing in their zones of control.

"We're making recommendations to the people who to vote for," said paramilitary commander Salvatore Mancuso, predicting the candidates they back would win 30% of the congressional seats.

The government is taking no chances for Sunday's vote.

Campaigning will become intense for May's presidential elections

In a massive security operation codenamed "Plan Democracy", more than 150,000 troops and heavily armed police will be mobilised to guard voting areas.

President Andres Pastrana called on Colombians to come out in force and use their vote.

"The best way to defeat violent groups is by casting ballots," he said in an address to the country's 24 million voters.

But whatever happens on Sunday, when the results are announced, nobody will expect any revolutionary change - the congress, forever enmeshed in corruption scandals, will plod along as always.

The elections that really matter - those for the next president - begin on 26 May.

The leading candidate for that post, Alvaro Uribe Velez, is hated by FARC guerrillas who have already tried to kill him, twice.

The election campaign is yet to really hot up.