Scottish Labour has been savouring a double by-election success and claimed it had delivered a blow to nationalist leader Alex Salmond.
Jim Devine retained Livingston for Labour with a reduced majority
The party retained the Livingston seat at Westminster and Holyrood's Glasgow Cathcart with reduced majorities.
Scottish Secretary Alistair Darling said the SNP would soon realise that Mr Salmond could not deliver.
The SNP registered an increased voting share in both polls and insisted it was on course to win power at Holyrood.
Labour retained the Livingston seat in West Lothian, which was made vacant on the death of former foreign secretary Robin Cook. However, its majority fell from 13,000 at the general election to 2,680.
Jim Devine, who was Mr Cook's election agent polled, 12,319 votes and he had a 2,680 majority over the SNP's Angela Constance.
Former Glasgow City Council leader Charlie Gordon held onto the Glasgow Cathcart seat and again the majority was reduced - from 5,112 in May 2003 to 2,405 on Thursday.
The seat became vacant following the conviction of former peer Mike Watson for wilful fire-raising at an Edinburgh hotel.
There was a record low turnout of 31.97% for the Holyrood by-election and that, accompanied by the slump in Livingston from 58.1% at the general election to 38.6%, has renewed concerns about voter apathy.
Labour, however, was keen to focus on the spoils of victory.
At a victory media conference in Glasgow, Mr Darling said: "The nationalists are on a downward slide under Salmond's leadership.
"The big story of last night is the result of the nationalists who fancy themselves as by-election specialists.
Charlie Gordon is congratulated by his wife Emma
"Alex Salmond's bluster of another Govan or Hamilton or of winning another 20 first-past-the-post seats at the Scottish election (in 2007) is now proved to be nothing more than empty bluster.
"The truth is, the nationalists are on a downward slide under Alex Salmond's leadership."
The SNP secured 9,639 votes in Livingston and 3,406 in Cathcart.
Mr Salmond pointed to a swing of 10% in favour of his party in Livingston which, if reproduced at the 2007 Scottish parliamentary elections, would deliver his party 28 seats.
He said: "We were the only party on the night to win votes in every seat, and that will give us the momentum we need for victory in 19 months' time.
"With the SNP on the move, we are the only party which can beat Labour and make a difference across Scotland."
Scottish Liberal Democrat Leader Nicol Stephen said his party's vote held up well in both seats despite the attempts of the SNP.
The Lib Dems were in third place with 15.4% in Livingston and improved from sixth to fourth in Cathcart.
Mr Stephen said: "These are solid results for the Liberal Democrats, holding on to our share of the vote in Livingston and improving on our previous result in Cathcart."
The Conservatives came fourth in Livingston with 4,499 votes and third in Cathcart with 2,306 votes.
Scottish Conservative leader David McLetchie said: "Our message is becoming more popular, we are improving our share of the vote, I'm convinced we'll continue to do so."
The Scottish Socialists polled in sixth place in Livingston and sixth in Cathcart, while the Greens were seventh in Cathcart and fifth in Livingston.