Hartshorn sought selection as a Labour candidate in the General Election of January 1910 in the Mid-Glamorganshire constituency. On this occasion he failed to gain the support of the South Wales Miners' Federation and had to stand down. He stood again as a Labour candidate in the April 1910 by-election, and yet again in the December General Election of the same year. On both these occasions he was backed by the South Wales Miners' Federation, but he was not successful.
During the National Coal Strike of 1912 there are references to Hartshorn in the press as a militant, an extremist, a socialist, as pugnacious, uncompromising and out for trouble . That said, he was not one of the group of young socialists from the Rhondda who published the Syndicalist manifesto, 'The Miners Next Step', with its sustained critique of the style of union leadership shown by the likes of William Abraham. Nor was he an advocate of conciliation and compromise, of co-operation with the coal-owners at any cost. He believed that the 'strike weapon' could and should be used.
The 1912 strike of almost 1 million miners was unprecedented. The Government, fearing widespread trouble on a much larger scale than the Tonypandy Riots of 1911, decided to intervene in the dispute. Within a month they had rushed a Minimum Wage Bill through Parliament. The resulting arrangements for miners' pay were, in operation, far from perfect, but the legislation did see the introduction of the first example of a national minimum wage.
A key player in the negotiations that took place in London, Hartshorn was less than enthusiastic about the Wage Bill, and he made it clear that it could not be accepted as a final solution to the problem. He took the view that if the men should decide to return to work, that would not be surrender, but simply a decision to test what they had won. The strike ended on 6 April 1912.
The production of coal in Britain peaked in 1913, but the First World War brought new challenges within the industry. Throughout the War there was a constant struggle between the collieries and the army for manpower, and conscription did more than any other issue to divide the Labour movement. Also, with the War came inflation but the miner's pay failed to keep pace with the rising cost of living. It became difficult for the miners' leaders to balance the interests of the miners and the interests of the nation. In March 1915 the Miners Federation of Great Britain demanded a 20% pay increase. The coal-owners refused to discuss a national wage rise and negotiations reverted to the districts. Satisfactory agreements were reached in most areas except South Wales where the obdurate coal-owners refused to budge beyond 10%. On 15 July 1915 about 200,000 miners in Wales went on strike. Hartshorn, and other Federation leaders, had urged the miners to abandon their plans to strike, and to continue negotiating while working. He reasoned that there would be a lack of public support for a strike while the country was at war, and naval defence depended on coal. There were also risks to the credibility of the Federation in the longer term. Inevitably he was criticised by some for 'betraying' the miners. The Coal Strike of 1915 was divisive, some viewed the miners as unpatriotic whilst others criticised the mine owners. Either way, it was a serious event, and the Minister for Munitions, Lloyd George, invoked the Munitions of War Act, making the strike illegal. He also travelled to Cardiff to speak to the miners directly. They remained defiant and won many of their demands before returning to work.
In November 1916 another strike over pay took place in Wales. This time the Government took control of the industry, and it was to remain under Government control for the next four years.
During the War Hartshorn visited the Western Front with other miners' leaders, ostensibly to assess morale and conditions among the troops. He became a member of the Coal Mining Organisation Committee (CMOC) established in 1915, and later a member of the Advisory Board to the Controller of Mines. He was also one of the three Wales Commissioners on the 1917 Inquiry into Industrial Unrest, and a member of the Colliery Recruiting Court in South Wales.
In January 1918 he was awarded the OBE in recognition of his service to the CMOC.This was the same month and year he lost his brother in Belgium to fighting. He visited the grave with his son not long after the finish of World War one and brought home photos of a simple wooden cross at the site. Th
Prior to, and throughout the War years, the campaign for women's suffrage continued, and it was a campaign that Hartshorn supported.He supported his daughter whilst she campaigned for Women's rights.