Scott Brown is determined to make his presence felt in the Senate
Voters in the US state of Massachusetts have elected Republican Scott Brown to the Senate seat long held by the late veteran Democrat Edward Kennedy.
Mr Brown's victory, which comes exactly a year after President Barack Obama swept into office on an agenda of change, is set to reverberate well beyond Massachusetts and even the US.
Why is Scott Brown's victory so important when Democrats have a majority in the Senate?
Mr Brown's win deprives the Democrats of their 60-40 super-majority in the Senate. This is needed to overcome any Republican moves to block legislation.
So given the current partisan nature of Congress, much of President Obama's agenda seems likely to be in doubt or be mired in difficult negotiations in Congress.
Key midterm congressional elections in November, when all seats in the House and a third of the Senate are up for grabs, complicate the picture. Politicians from both sides will be reluctant to vote for issues that may cost them electorally.
Which key policy areas are likely to be affected in the wake of Mr Brown's win?
Most attention in the US is on healthcare reform, Mr Obama's main domestic priority. He had hoped to have a bill in place before delivering his state of the union address, now set for 27 January.
The House of Representatives and the Senate spent several months in 2009 crafting their bills amid often bitterly contested debates.
is highly controversial and votes on the issue in the House and the Senate were overwhelmingly along party lines.
two versions of the legislation,
which have some significant differences, were being merged into a single bill that both houses of Congress could then approve and send to Mr Obama to be signed into law.
For that the Democrats needed 60 Senate seats but now the Democratic caucus is one seat shy of the super-majority.
campaigned on a promise to be "the 41st senator", the one whose vote would allow the Republicans to use procedural moves known as a filibuster to block Mr Obama's healthcare bill.
So what options do the Democrats and Mr Obama have to save their health reform proposals?
Not many and none ideal.
One would be to try to use the Democratic majorities in both houses and rush a compromise bill through before Mr Brown can take his seat. But not only is Mr Brown likely to be in the Senate quickly, such a move would provoke an outcry from Republicans and possibly a backlash from voters.
Another would be for the House of Representatives to pass the Senate's version as it is. That would allow a final bill to be sent to Mr Obama without the legislation going again before the Senate.
That is unlikely given the degree of differences between the two bills. Many Democrats in the House would be very reluctant to go along with this.
Another variation is for the Senate bill to be passed as the final healthcare bill and then subsequently amended in a process that only needs a 51-vote majority. That, however would be tricky, technical and time-consuming, and delay the rest of the legislative agenda.
Or, the large-scale plans for reform could be shelved and a version drawn up that includes some of the most popular elements in the existing bills.
However, Democratic leaders point to the fact that Massachusetts already has near universal health coverage and therefore the decision of voters there should not determine whether the rest of the US should have such a system.
What other areas of Mr Obama's agenda are vulnerable?
Perhaps the most important globally are moves on climate change. The Senate has yet to pass a bill to establish a "cap and trade" scheme to cut greenhouse emissions. The House passed its version last year.
The Democrats were already struggling to get legislation through the Senate and the loss of a seat makes this all the harder. In addition, some Democrats from traditional manufacturing states are increasingly reluctant to vote for legislation seen as threatening US jobs.
So there will either be no bill or one that is watered down. That may then provoke a backlash from other
big polluting countries that are considering their own emissions cuts.
Is Mr Brown's success an early indicator of what will happen in November?
The Democrats' failure to hold onto such a seemingly impregnable seat as Massachusetts is embarrassing and
its impact has been widely discussed by political commentators
Republicans say the victory of Mr Brown, who voiced opposition to the healthcare legislation, is a sign that Democrats who did well in 2006 and 2008 midterms have misread the voters' mood.
Traditionally, the president and his party fare badly in midterms.