By Stephen Mulvey BBC News, Ukraine

Yushchenko and Tymoshenko appeared to be the best of friends

An Orange Revolution memorabilia stall on Kiev's chestnut-lined main street, the Khreshchatyk, says quite a lot about the current state of Ukrainian politics.

A t-shirt depicting the revolution's figurehead, Viktor Yushchenko - now the country's president - has stopped selling and will not be re-ordered, says saleswoman Viktoria Kucherenko.

But a t-shirt of the revolution's leading lady, Yulia Tymoshenko - recently sacked as prime minister by Mr Yushchenko - is selling briskly.

To be fair, the Yushchenko shirt is an older product and the Tymoshenko shirt is more popular partly because it is newer and more topical.

But the way the two of them are going head-to-head in the shirt market mirrors the personal rivalry that has now come out into the open and which could be Ukraine's big political story for years to come.

The Tymoshenko shirt carries a quotation from the first television interview she gave after her dismissal: "I'm not going to hide myself away - I'm not going anywhere."

Secret pact

She aims to get her job back by winning the parliamentary elections in March.

The Tymoshenko shirt (left) is the biggest seller

The result would be an uncomfortable "co-habitation", of the kind that sometimes occurs in France, between a president and a prime minister who do not see eye-to-eye.

However, in this case the differences would be on a personal more than political level.

Rank and file supporters of the revolution were dismayed when the ex-comrades-in-arms fell out in early September and the government was dismissed.

Mr Yushchenko's illness in 2004 meant that Ms Tymoshenko had taken on the full burden of the election campaign, criss-crossing the country to promote his candidacy.

It was her fiery oratory that helped draw the crowds back to the demonstration on Kiev's Independence Square again and again in the freezing winter temperatures.

UKRAINE TIMELINE 31 Oct 2004: first round of presidential election 21 Nov: second round, mass demonstrations begin 3 Dec: Supreme Court annuls second round results 26 Dec: decisive round of elections won by Viktor Yushchenko 23 Jan 2005: Yushchenko sworn in as new president 4 Feb: Yulia Tymoshenko named new prime minister 8 Sep: Yushchenko sacks government Battle for Ukraine

So when people voted Yushchenko, many were probably voting for a package - Yushchenko and Tymoshenko, and their promise of a new free, democratic Ukraine.

But according to one of the country's best-connected journalists, Yulia Mostova, editor of Dzerkalo Tizhnya (Mirror of the Week), the couple never liked each other and only joined forces for the election because they knew that otherwise both would lose.

In September 2004, Ms Mostova says, they signed a coalition pact, which had a secret protocol promising Ms Tymoshenko the premiership in case of victory.

But when they won the election, she says, Mr Yushchenko tried to persuade Ms Tymoshenko to release him from this promise.

"She said: 'If you don't want to fulfil your obligations, don't. But I won't give in.' [As prime minister] she expected to be sacked every day, because she understood the reality of their relationship - hatred," says Ms Mostova.

Aiming high

There is speculation that the pair could put aside their differences again to form another bloc, either before or after the parliamentary election.

But Ms Tymoshenko's campaign manager, Oleksandr Turchynov, until recently head of the Ukrainian security service, says there is no chance of this unless the authorities prosecute businessmen and officials he has accused of corruption, including close allies of Mr Yushchenko.

People in Kiev assess progress since the Orange Revolution In pictures

As he puts it: "They must return to promises we made together on the Maidan [Independence Square], to throw out people suspected of corruption and to halt the influence of oligarchs on politics."

Ms Mostova's worry is that the rival Orange factions will "drive each other into the grave, leaving their former enemy without opponents".

Mr Yushchenko's and Ms Tymoshenko's natural supporters could be so disheartened that a large proportion of them will not vote, she suggests, while other voters go to the polls in force to ensure a parliamentary victory for the losing presidential candidate, Viktor Yanukovych.