When a group of employees get together to form a union in the US, the first thing they do is pass around membership cards the the other employees at the company/employing unit, and try to get people to sign on.  If they can get a majority of the employees to sign the cards positively, then they can take the cards to the employer and demand an election.

You see, having the statement of intent is not enough.  They then have to pass an election, again with 50% of the employees voting.

When is the election?  The employer gets to determine that.  There is a government body, the National Labor Relations Board, that exists to arbitrate such disputes, but one can imagine how impartial it is under Bush.

The election system is a huge problem, because the employer is free to bombard its workers with anti-union propaganda at all times of the day, whereas union supporters can only canvass during break times.  Further, it's common practice for the union activists (and like any political activity, forming a union is usually driven by a few dedicated individuals giving shape to a mass of unformed discontent) to be harassed, transferred, reassigned, or fired for spurious reasons - whatever it takes to keep them from agitating for the union.  Employers regularly will hold mandatory propaganda sessions to convince their employees of the evils of unions - meetings at which the pro-union forces cannot speak, but meetings which they nonetheless must attend.

After all this, there is the election.  Various vote-suppression and vote-rigging techniques are quite common, as it is the employer who runs the election and counts the ballots.

Then, if the union wins, the employer can challenge the vote in a variety of ways, ultimately taking it to the NLRB.  This process can take quite a while, time the employer still can use to oust the union activists.

Once the election victory has been certified, the union is properly established, and it's time for collective bargaining for a new employment contract.  This begins when the employer decides it begins.  It lasts as long as the employer decides it should last.  Usually, it takes strikes to get a contract, even one that was no different from what was had beforehand.  Because, unless the union can get a first contract that actually means something, it's still weak.  If the union fails at this stage, it usually fails forever.

In case of a union victory, it then has the privelige of re-negotiating the contract every two or three years.  While these are always opportunities for the union to improve the contract, they are also opportunities for the employer to crush the union - again, if the union can't get a contract, or discontent with the harassment brought on by union activities grows strong enough, it's always possible for the employer to push to de-certify the union, forcing another election, under the same terms as before.